The present invention relates to dampening systems for offset presses, and more particularly relates to dampening systems in which the speed of the dampening system is different than the speed of the press system.
In prior offset presses, the rolls of the dampening system have operated at a velocity less than the rolls of the press system. This speed differential results in the formation of a slipping nip at the interface of the dampening system and the press system. A slipping nip, i.e. a nip at which the contacting surfaces of the opposed rollers slip against one another, is desirable in high-speed offset presses to achieve proper application of dampening fluid to the press rolls.
A prior art printing press system is shown, for example, by FIG. 1. A printing system 1 includes a dampening system 2 and a press system 3. The dampening system 2 includes a pan roll 5, which receives dampening fluid from a dampening fluid reservoir 4. A slip roll 6 is engaged with pan roll 5, and both pan roll 5 and slip roll 6 are driven to rotate at the same surface velocity (i.e. the linear speed at the surface of each roll is equal). The rotation and contact of the pan roll 5 and slip roll 6 thus cause the dampening fluid carried by pan roll 5 to be tranferred to slip roll 6.
Press system 3 includes a press roll 7, which is engaged typically via non-slipping nips with subsequent rolls in the press system (not shown). Press roll 7 also engages slip roll 6, and by such contact press roll 7 receives dampening fluid carried by slip roll 6. Press roll 7 will thereafter transfer the dampening fluid to the further elements of the press system. Press roll 7 is driven to rotate at a surface speed greater than surface speed of slip roll 6, thus causing a slipping nip 8 at the interface of the dampening system 2 and press system 3 to deliver dampening fluid to the press system 3.
It has been observed that fluid transfer across a slipping nip deteriorates when the speed ratio between the two engaged rolls (i.e. surface speed of fast roll: surface speed of slow roll) is large. In prior systems, in order to reduce the speed ratio at the slipping nip without reducing the press system speed, the speed of the dampening system was increased. However, this increase in the speed of the dampening system resulted in further problems. For example, at high dampening system speeds excessive amounts of fluid may be delivered to the press system. Furthermore, at high dampening system speeds dampening fluid may be thrown from the dampening system rolls, resulting in inconsistent delivery of dampening fluid.